In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for transferring, on a Si substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional photolithography method has been close to the resolution limit, while microsizing of semiconductor devices has been accelerated. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF laser, is considered to be promising as an exposure technique for 45 nm or below. In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. For this reason, for EUV light lithography, a catoptric system, i.e. a combination of a reflective photomask (hereinafter referred to as EUV mask) and mirrors, is employed.
A mask blank is a film-laminated plate for a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a reflective film for reflecting EUV light and an absorber film for absorbing EUV light, are formed in this order on a substrate made of glass or the like (Patent Document 1). Besides these films, in such an EUV mask blank, a protection film for protecting the reflective film at a time of forming a mask pattern in the absorber film, is usually formed between the reflective film and the absorber film. Further, on the absorber film, an antireflective film is usually formed for improving the optical contrast at a time of inspecting a mask pattern.
In such an EUV mask blank, the thickness of the absorber film is preferably thin. In EUV lithography, exposure light is not incident from a perpendicular direction to an EUV mask but incident from a direction at an angle of a few degrees, usually 6 degrees, to the perpendicular direction. If the thickness of the absorber film is thick, at a time of EUV lithography, a shadow of the exposure light is formed on a mask pattern formed by removing a part of the absorber film by etching, and the pattern accuracy or the dimension accuracy of a mask pattern (hereinafter referred to as “transfer pattern”) transferred to a resist on a substrate such as a Si wafer by using the EUV mask, tends to be deteriorated. Since this problem becomes more significant as the line width of the mask pattern formed on the EUV mask becomes smaller, the thickness of the absorber film of the EUV mask blank is required to be thinner. However, in order to maintain absorptivity of EUV light, the absorber film needs to have a certain thickness.
For the absorber film of the EUV mask blank, a material having a high absorption coefficient for EUV light is employed, and the thickness is ideally a thickness whereby EUV light incident into a surface of the absorber film is completely absorbed. However, as described above, since the thickness of the absorber film is required to be thin, it is not possible for the absorber film to completely absorb EUV light incident into the layer, and a part of the incident light will be reflected from the layer.
The step of forming a transfer pattern of a resist on a substrate by EUV lithography requires the optical contrast of reflected light from the EUV mask, that is, the contrast between reflected light from a portion of the mask wherein the absorber film is removed at a time of forming the mask pattern so that the reflective film is exposed to the outside, and reflected light from a portion of the mask wherein the absorber film is not removed at the time of forming the mask pattern. Accordingly, it has been considered that so long as a sufficient optical contrast of reflected light is obtained, there is no problem even if the incident EUV light is not completely absorbed by the absorber film.
Based on the above concept, in order to reduce the thickness of the absorber film, an EUV mask using the principle of phase shift is proposed (refer to Patent Document 2). This has a characteristics that a portion from which the absorber film is not removed at a time of forming a mask pattern has a reflectivity of from 5 to 15% for EUV light (reflected light), and that EUV reflected light from such a portion has a phase difference of 175 to 185° from EUV reflected light from a portion where the absorber film is removed at the time of forming the mask pattern so that the reflective film is exposed to the outside. The document describes that with the EUV mask, by using the principle of phase shift for reflected light from the absorber film, it is possible to maintain a sufficient optical contrast to the reflective film, and accordingly, it is possible to reduce the thickness of the absorber film.